On 11/22/25 10:03, Barry Song wrote:
From: Barry Song <[email protected]>
In many cases, the pages passed to vmap() may include
high-order pages—for example, the systemheap often allocates
pages in descending order: order 8, then 4, then 0. Currently,
vmap() iterates over every page individually—even the pages
inside a high-order block are handled one by one. This patch
detects high-order pages and maps them as a single contiguous
block whenever possible.
Another possibility is to implement a new API, vmap_sg().
However, that change seems to be quite large in scope.
When vmapping a 128MB dma-buf using the systemheap,
this RFC appears to make system_heap_do_vmap() 16× faster:
W/ patch:
[ 51.363682] system_heap_do_vmap took 2474000 ns
[ 53.307044] system_heap_do_vmap took 2469008 ns
[ 55.061985] system_heap_do_vmap took 2519008 ns
[ 56.653810] system_heap_do_vmap took 2674000 ns
W/o patch:
[ 8.260880] system_heap_do_vmap took 39490000 ns
[ 32.513292] system_heap_do_vmap took 38784000 ns
[ 82.673374] system_heap_do_vmap took 40711008 ns
[ 84.579062] system_heap_do_vmap took 40236000 ns
Cc: Uladzislau Rezki <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Maxime Ripard <[email protected]>
Signed-off-by: Barry Song <[email protected]>
---
mm/vmalloc.c | 49 +++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 43 insertions(+), 6 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 0832f944544c..af2e3e8c052a 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -642,6 +642,34 @@ static int vmap_small_pages_range_noflush(unsigned long
addr, unsigned long end,
return err;
}
+static inline int get_vmap_batch_order(struct page **pages,
+ unsigned int stride,
+ int max_steps,
+ unsigned int idx)
These fit into less lines.
ideally
\t\tunsigned int stride, int max_steps, unsigned int idx)
+{
int order, nr_pages, i;
struct page *base;
But I think you can just drop "base". And order.
+ /*
+ * Currently, batching is only supported in vmap_pages_range
+ * when page_shift == PAGE_SHIFT.
+ */
+ if (stride != 1)
+ return 0;
+
+ struct page *base = pages[idx];
+ if (!PageHead(base))
+ return 0;
+
+ int order = compound_order(base);
+ int nr_pages = 1 << order;
You can drop the head check etc and simply do
nr_pages = compound_nr(pages[idx]);
if (nr_pages == 1)
return 0;
Which raises the question: are these things folios? I assume not.
+
+ if (max_steps < nr_pages)
+ return 0;
+
+ for (int i = 0; i < nr_pages; i++)
+ if (pages[idx + i] != base + i)
+ return 0;
if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
return compound_order(pages[idx]);
return 0;
--
Cheers
David
